An optical emission analyzer is a device for determining the types of elements contained in a sample to be analyzed and the content of each element by exciting the sample into a light-emitting state, separating the emitted light a light-dispersing element or similar device into spectral lines characteristic of the constituent elements using, checking for the presence of each spectral line and, if the line is present, measuring its intensity. One method for causing an excitation emission of a sample to be analyzed is to generate a spark discharge within a space (or discharge gap) between the sample and a discharge electrode to simultaneously perform both the vaporization and excitation of the atoms on the sample surface by a discharge plasma (refer to Patent Document 1).
FIG. 5 is a circuit diagram showing the configuration of a conventional optical emission analyzer. This optical emission analyzer is composed of a capacitor circuit 1, igniter circuit 2, emission stand 3, photometric circuit 4 and arc-generating circuit 5. In the emission stand 3, a discharge electrode 31 and a sample 32 are provided. The igniter circuit 2, capacitor circuit 1 and arc-generating circuit 5 are connected in series to the discharge electrode 31 and the sample 32. The capacitor circuit 1 includes a capacitor-charging circuit 11, rectifying diode 12, capacitor 13 and clamp diode 14. The igniter circuit 2 has an igniter transformer 21 and igniter drive circuit 22.
The capacitor-charging circuit 11 charges the capacitor 13 to a predetermined voltage via the rectifying diode 12. After the charging of the capacitor 13 is completed, the igniter drive circuit 22 generates a high voltage in a secondary winding of the igniter transformer 21 to initiate an electric discharge between the discharge electrode 31 and the sample 32. Consequently, a spark current (discharge current) flows through the current path formed by the capacitor 13, igniter transformer 21, emission stand 3 and bypass diode 54, with the energy charged in the capacitor 13 being transferred into the space between the discharge electrode 31 and the sample 32 to create a plasma.
Meanwhile, in the arc-generating circuit 5, a switching element 52 is connected to a power source 51 simultaneously with the beginning of the spark discharge, to initiate excitation of a coil 53. The excitation current of the coil 53 increases when the switching element 52 is connected to the contact to the power source 51, and decreases when the switching element 52 is connected to the common contact. The switching action and frequency of the switching element 52 are controlled so as to maintain the excitation current of the coil 53 at a predetermined target value.
FIGS. 6 and 7 is a graph showing the relationship among the discharge current Id in the emission stand 3, the excitation current Ia in the coil 53 and the output voltage Va of the arc-generating circuit 5 in the case where the target value of the excitation current of the coil 53 is 10 A. After the spark discharge is initiated, the discharge current Id rapidly increases and then decreases with time. Meanwhile, the excitation current Ia gradually increases after the excitation of the coil 53 is initiated.
After the initiation of the spark discharge and excitation of the coil 53, the discharge current Id flows through both the coil 53 and bypass diode 54 during a period of time where the discharge current Id exceeds the excitation current Ia. When the discharge current Id becomes equal to the excitation current Ia, the bypass diode 54 turns off, after which only the excitation current Ia flows through the emission stand 3. As a result, the discharge between the discharge electrode 31 and the sample 32 changes to an arc discharge. The arc discharge between the discharge electrode 31 and the sample 32 is maintained while the switching element continues its switching action.
As shown in FIG. 6, if the excitation current Ia in the coil 53 reaches the target value within the duration of the spark discharge, a smooth transition is made from the spark discharge to the arc discharge. On the other hand, if, as shown in FIG. 7, the excitation current Ia in the coil 53 fails to reach the target value within the duration of the spark discharge, the waveform of the discharge current will be distorted (similar to the portion surrounded by broken line A in FIG. 7).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-300630